I believe you have a electric pto on that model.
When it's hot, shut down motor. Turn key back on so you have power, engage pto.
You should hear a "click" when it engages.
Sounds like a connection is braking sometimes when it's hot.
If it was a short it would pop the fuse and not work at all.
Cubs usually have a switch on the reverse pedal rod. There's a metal tab that pushes on switch to prevent reverse mowing. Bend tab to one side to miss switch.
